chore: migrate to pyproject.toml; drop Python 3.8/3.9 support

[tseporamaisa]

Summary

• DAT-155: migrate setup.py to PEP 621 pyproject.toml, setup.py reduced to a minimal stub. license = "ISC" (matches the actual LICENSE file text and classifier, not the free-text "ISCL" setup.py used).
• Bumped to Python 3.13. Real CI matrix was 3.8/3.9 (GitHub Actions, tox.ini, classifiers all agreed). This package is installed into the sagemaker/Dockerfile of ~8 repos across the fleet at a range of Python versions (3.9-3.11 currently) — not a breaking change for them (pip just keeps older consumers on the last compatible ml2p release), but flagged given the unusually wide blast radius; confirmed going straight to 3.13 to match the rest of this migration. setuptools>=83.0.0.
• Migrated tests/cli_commands/test_utils.py's pkg_resources.resource_filename usage to importlib.resources (deprecated, slated for removal from setuptools).
• Minor version bump (0.6.0 → 0.7.0) + HISTORY.rst entry, per this org's convention for a Python-support change.
• No Dockerfile/PyPI-credential concerns — pure library, no Docker build of its own, published via GitHub Actions to public PyPI (unaffected).

Three real, pre-existing test bugs found and fixed — surfaced only by running on 3.13 against fresh (unpinned) dependency versions, none are new 3.13 incompatibilities in ml2p's own code:

• fake_datetime.now() only accepted **kwargs, but newer botocore calls datetime.now(datetime.timezone.utc) positionally — now accepts (and ignores, to stay deterministic for other callers) a tz param.
• assert_traceback() did an exact regex match that doesn't tolerate the PEP 657 fine-grained error location markers Python 3.11+ adds — now strips those lines first.
• test_create_and_list asserted an exact CreationTime/LastModifiedTime that actually comes from moto's own internal clock (unreachable by the fake_utcnow fixture, which can't patch moto's already-imported datetime reference) — a newer moto returns a tz-aware local-offset value instead of the old naive one. Now only asserts the fields are present.

Test plan

• Local Python 3.13.5 venv: 210/210 tests pass (1 skipped)
• black/isort/flake8 clean
• Both ml2p and ml2p-docker console scripts work

[KyleHarrison]

LGTM